Streptococcus agalactiae pulsed-field gel electrophoresis patterns cross capsular types

Abstract

Streptococcus agalactiae is a genetically diverse organism; when typed by pulsed-field gel electrophoresis (PFGE), multiple types appear within a single serotype. We tested whether S. agalactiae PFGE types correspond to a specific serotype within individuals, and different individuals from the same geographic area. A total of 872 S. agalactiae isolates from 152 healthy individuals were classified by PFGE and capsular serotype. Serotype V was the most homogeneous (Simpson's diversity index 0.54); and types III, II and Ib were mostly heterogeneous (Simpson's diversity index 0.90). Within an individual, isolates with the same PFGE patterns had identical capsular types, but across individuals the same PFGE types sometimes occurred in different serotypes. Capsular type alone is insufficient to define epidemiological relatedness. Although PFGE types appear to be a valid surrogate for capsular typing of isolates from the same individual, it is not a valid surrogate for serotype in isolates from different individuals.

Fig. 1

Capsular-type distribution of S. agalactiae isolates collected between January and April 2001, from healthy male and female University of Michigan undergraduates based on the total collection (■, n=872) and a subgroup collection of one isolate for each unique PFGE pattern from an individual (□, n=168). NT, Non-typable.

Fig. 2

Representative PFGE pattern of 12 colonizing S. agalactiae isolates recovered from two healthy individuals: Group B Streptococcus recovered from throat, rectal and urine samples of study participant no. 760 are shown in lanes 2–7 and 9–10, and from study participant no. 774 in lanes 11–14. Lanes 2–5, 7 and 9–10 from participant no. 760 were capsular type III, while lane 6 was capsular type II. Lanes 11–14 from participant no. 774 were capsular type V. Lanes 1, 8, 15 are control strain A909.